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Saturday, October 23, 2010

Deceptive beauties take two

Orchids use a number of strategies to lure in pollinators. Besides docilely offering nectar like most other flowers, they often mimic the scents or appearance of other insects, particularly females of the pollinating species. Johannes Stökl from the Max-Planck Institute and his colleagues from Max-Planck, Haifa University and the University of Ulm have added a new kind of orchid deception to the list. Epipactis veratrifolia lures predatory insects by producing the alarm pheromones of their prey.

Hoverflies need to lay their eggs on plants containing aphids, the food source for their young. When aphids are in danger, they produce specific chemicals to warn other aphids away from the area. Hoverflies hone in on these signals and lay their eggs nearby. E. veratrifolia can make these same chemicals, inducing female hoverflies to approach close enough to collect and transfer pollen. Male hoverflies are also coerced into helping the plant as they hopefully wait for females.

The researchers suspect that the orchids first used the aphid pheromones as a way to prevent aphids from eating them. The plants are remarkably free of aphids, so this does seem to be an effective strategy. Only later were the same chemicals used to attract pollinators. There are a couple of lines of evidence for this. E. veratrifolia offers its pollinators a small amount of nectar, indicating that it doesn’t rely entirely on its deceptive tactics. Also, by luring the hoverflies to lay their eggs on aphid-free flowers, the orchids are ensuring the death of the next generation of their own pollinators, an evolutionarily unstable relationship.

Caption: Eastern marsh helleborine (Epipactis veratrifolia), an orchid species, has successfully lured a hoverfly of the genus Ischiodon by mimicking alarm pheromones usually emitted by aphids.

Credit: MPI Chemical Ecology, Johannes Stökl